This invention relates to turbocharged compression ignition engines having exhaust gas aftertreatment systems and, more particularly, to methods of controlling the turbocharger to regulate the exhaust gas oxygen content and temperature.
High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Turbocharger power assist systems (TPAS) allow the intake airflow to be optimized over a range of engine speeds by electronically driving the compressor. Such systems have typically been used to improve engine performance. During engine acceleration from low engine speed and load conditions, the TPAS can be used to add a positive torque on the turbocharger shaft thereby improving turbocharger acceleration. Faster response of the turbocharger results in faster increase of the fresh air charge delivery to the engine. Since the amount of fuel that can be burnt in a diesel engine without generating visible smoke is limited by the amount of fresh air charge, faster rise in the air charge delivery results in the improved diesel engine acceleration performance. In particular, an important transient performance characteristic of turbocharged engines, the so called turbo-lag, can be reduced. At higher engine speed and load conditions, the TPAS can be used to absorb and store (e.g., in a battery) some of the excess energy provided by the exhaust gas to the turbocharger, essentially, emulating a wastegate and preventing engine overboost. This energy is normally be wasted as a portion of the exhaust gas bypasses the turbocharger when the wastegate opens.
The inventors herein have discovered that the effect of applying a positive power to the turbocharger shaft results in a decrease in the post-turbine exhaust gas temperature and increase the post-turbine exhaust gas air/fuel ratio. Conversely, the application of negative power to the compressor results in increased temperature of the post-turbine exhaust gas and decrease in the exhaust gas oxygen concentration. Both the temperature of the exhaust gas and the oxygen concentration in the exhaust gas are important variables in diesel engine aftertreatment control.
One object of the present invention is to provide an improved compression ignition engine control strategy.
Another object is to provide improved diesel aftertreatment control.
According to the present invention, the foregoing and other objects and advantages are obtained by According to the present invention, the foregoing and other objects and advantages are attained by a method of controlling an exhaust gas aftertreatment system in a compression ignition engine having a power-assisted turbocharger coupled to a power source. The turbocharger includes a compressor in communication with an intake manifold and a turbine in communication with an exhaust manifold. The method comprises the steps of determining an operating mode of the exhaust gas aftertreatment system selected from the group consisting of a light off mode, regeneration mode, and storage mode. The method then determines a charge value associated with the power source, and engages the power source to the power-assisted turbocharger as a function of the charge value and the operating mode of the exhaust gas aftertreatment system.
In one aspect of the invention, negative power is applied to the power-assisted turbocharger to increase the exhaust gas temperature to achieve particulate filter light-off. In another aspect, positive power is applied to the power-assisted turbocharger to reduce turbo-lag or engine pumping losses. In another aspect, negative or positive power is applied to the power-assisted turbocharger to maintain a desired oxygen flow through the particulate filter.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.